1. Field of the Invention
The present invention relates to a dielectric-laminate type band-pass filter for use in a portable radio or the like in the frequency range of several hundred MHz to several GHz.
2. Description of the Prior Art
Conventional resonators are divided roughly into the resonator using a strip line and the resonator using a coil pattern. A band-pass filter is constructed using such resonators, by connecting plural resonators magnetically.
A resonator using the strip line, includes a resonator of 1/2 wavelength whose line is open at both ends as shown in FIG. 36, and a resonator of 1/4 wavelength whose line is open at one end and shortcircuited at the other end as shown in FIG. 37.
A resonator using the coil pattern, as shown in FIG. 38, includes one in which a spiral-shaped coil pattern 201 and an earth pattern 203 are formed on both sides of a dielectric layer 202 which is clamped therebetween.
In the band-pass filter using the above-mentioned conventional resonator, however, such problems as shown hereinafter were encountered respectively.
[1] band-pass filter using the strip line
(a) A resonator having the resonance frequency of 2 to 3 GHz becomes substantially large. In particular, a band-pass filter having a construction in which plural resonators are connected, becomes considerably large. This is due to the following reasons. PA1 (b) In the band-pass filter, it is desirable to adjust input/output impedances depending on the apparatus into which it is incorporated (the adjustment involves matching of impedances of the band-pass filter and the apparatus). However, in case of the strip-line type, since input/output impedances have specific values for every strip line, it is impossible to adjust the matching even by changing the take-out position from the strip line.
That is, lengths L.sub.10, L.sub.11 of the strip line are determined as shown in Equation 1 (resonator of 1/2 wavelength) and Equation 2 (resonator of 1/4 wavelength).
Equation 1 ##EQU1##
Equation 2 ##EQU2##
Where, .lambda.; wavelength, .epsilon.; dielectric constant of dielectric-laminate sheet.
At present, a dielectric constant of a dielectric-laminate sheet which is capable of being co-fired with silver or copper and has a good temperature characteristic can not be made so large, about .epsilon..apprxeq.10. Thus, in the above Equations 1 and 2, when .epsilon.=10, the dimensions L.sub.10 =15.8 mm and L.sub.11 =7.9 mm, which are very long, thus resulting in a large resonator (band-pass filter) as stated above.
[2] band-pass filter using the coil-pattern
Since the coil pattern has a spiral shape, magnetic fluxes influence one another between the adjoining patterns, thus an electric current flow is difficult to obtain. Therefore, a substantial resistance increases and the Q value becomes lower.
For example, in FIG. 38, since the electric current flows in a same direction (both in the direction D in FIG. 38) in a pattern piece 201a and a pattern piece 201b, magnetic fields cancel each other to cause the magnetic flux to become coarse, and consequently an electric current flow is disturbed and the substantial resistance increases.
A further problem is that an insertion loss of the band-pass filter becomes larger to the extent the Q value is reduced.
The passing band frequency of the band-pass filter depends on the resonance frequencies of the resonators. The resonance frequencies of the resonators are decided by the dimensions of strip lines or coil-patterns, so once these patterns are formed, the resonance frequencies can not be adjusted. Therefore, when the dimensions of the patterns are in error, the passing band frequency of the band-pass filter is shifted from a predetermined desired value, resulting in an inferior device.